1. Field of the Invention
The present invention relates to continuously variable ratio transmissions (CVT) employing planetary gear sets, and a system in which the CVT is integrated with the input driver and output driven systems through a regenerative/control subsystem. The present also relates to varying forms of the planetary gear set used in the CVT.
2. Description of Related Art
Planetary gear sets are well known in the art as a form of a mechanical transmission device. The planetary gear set operates with three rotary elements rotating around at least three different axles. The three rotary elements are a sun gear, a ring gear, and a planet gear/gears carried on a planet carrier. The three elements are meshed together via gear teeth, for example.
FIG. 1, shows, schematically, an exploded view of a basic planetary gear set 10. The ring gear 12 has a cavity 14 into which the planet gears 16 and sun gear 18 are positioned. Planet gears 16, carried on planet carrier 20, mesh with an inner surface of the ring gear 12. Sun gear 18 meshes with the planet gears 16 at an inner extent. Sun gear 18 has an axle 22 extending through planet carrier 20, and a control gear 24 of planet carrier 20 may be driven at varying rates or locked in place, to provide the variation in the transmission ratio between the sun gear axle 22 and the ring gear axle 26.
FIGS. 2 A–C illustrate variants on the basic planetary gear set shown in FIG. 1, all of which can be suitable for use in a CVT. FIG. 2A illustrates a planetary gear set having multiple meshed planet gears. FIG. 2B illustrates planetary gear set employing compound planet gears, meaning that the planet gears at either end of the planet carrier are each actually made up of two gears of different diameters, the gears of one diameter engaging the ring gear, and the gears of the other diameter engaging the sun gear. FIG. 2C is a simple differential having at least three rotary elements that are accessible from coupling with axles outside the transmission system. One of the rotary elements can be subjected to external control to vary the transmission ratio of the other two rotary elements axles.
In most current industrial applications, one element or axle, for example, the ring gear/axle, work as the input axle. A second element or axle, for example, the sun gear/axle, works as the output axle. The third element and axle, e.g., the planet carrier with its planet gears, in this example, has a rotation control device such as a brake or clutch, to cause the other two elements to rotate at a fixed ratio of rotary speeds. The roles of the three gear/axle elements may be interchanged, and the foregoing is simply one example. Planetary gear systems have inherently high efficiency when used in transmission applications of this type.
U.S. Pat. No. 4,973,295, issued on Nov. 27, 1990, to the present inventor, discloses a stepless (continuously) variable ratio transmission (CVT) in which all three rotary elements are capable of rotating, and the rotation of the axle that is not the input or output axle is variably controlled to vary the transmission ratio between the other two elements. The disclosure in this patent is hereby expressly incorporated by reference.
In one of the embodiments disclosed in the '295 patent, the planet carrier is operated to control the transmission ratio between the sun gear/axle and ring gear/axle, by coupling the planet carrier to a worm gear which is driven at a desired speed (or held stationary, at one extreme) by the worm which in turn is driven by a motor such as a stepping motor or servomotor. The motor is controlled by a computer which monitors the speed of the input and output axles, to control the motor to drive the worm gear at a speed which will yield the desired rotary relationship (ratio) between the input and output axles.
Planetary gear systems and variations thereon are three (or more) terminal mechanical systems in which a larger flow of energy between the input and output is controlled by the lower energy control terminal. The ratio between the larger energy flow under control and the energy needed to provide such control is called control gain. For example, if a 1 hp servomotor is needed to control a CVT connecting between a motor and a water pump both with nominal power of 100 hp, then the control gain of the system is roughly around 100.
The range of CVT transmission ratios is the ratio between the highest and lowest transmission ratios at which a CVT is designed to work. For example, if a CVT works between 3:1 (ratio=input RPM:output RPM, therefore higher reduction has higher ratio value) and 1:1, then the ratio range is 3. If, on the other hand, a CVT works between 3:1 and 1:3 will have a ratio range of 9. The latter is defined to have a wider range (9) than the former (3).
The overall efficiency is important for evaluating a mechanical transmission system. A transmission needs energy to overcome friction, for energy conversion, and for accessories that are necessary for the functioning of the transmission system. The ratio between the energy available for output and the total energy input is the overall efficiency of the system.
In the CVT disclosed in the '295 patent, the progression angle of the worm—worm gear pair was chosen at the “critical angle” so the worm—worm gear is in between the self-locking and non-self-locking condition. In other words, the power needed to drive and control the rotation speed of the step-/servo-motor is minimized. Also, part of the energy needed to overcome the friction involved in the worm—worm gear is from the main energy source of the input axle.
When a CVT system, such as the one disclosed in the '295 patent, works within a ratio range, e.g., for reduction, the minimum reduction ratio is achieved by mechanical geometry design of the gear set while assuming the control axle is in standstill condition. The control axle has to spin faster when higher reduction ratio is required. Despite the mechanical gear system's inherent high efficiency, the overall efficiency will be the highest when the control axle is not moving, because the system does not need to overcome the friction involved in the worm system, nor is any energy needed to rotate the servomotor to drive the worm. When the control axle is moving faster at the upper range of the variable ratios, the overall efficiency will be lowered.
Thus, while a CVT of such design works in a wide range of transmission ratios, at the higher transmission ratio side of the range, the overall efficiency will be at the lowest.
In addition to efficiency concerns when operating at the higher end of transmission ratios, certain applications for which a CVT might be considered have requirement for low noise and the like, for which gear sets operating with meshed gear teeth may not be suitable.
Further, applications may exist in which the efficiency and control gain desired from a CVT can not be adequately met by a single planetary gear set.
It is therefore a principal object of the present invention to provide CVT system that improves upon the overall efficiency of the system in operation, particularly at the upper end of transmission ratios.
It is a further principal object of the present invention to provide low-noise CVT designs for use in applications requiring low noise operating conditions.
It is yet a further principal object of the present invention to provide a CVT systems having improved control gain and efficiency characteristics.